In the field of computed radiography, a photographic element has an image formed thereon by x-rays. The element is later stimulated to emit a radiation pattern that is indicative of the image formed by the x-rays. Typically, storage phosphors are used to capture radiographic images from incident x-rays. Most radiographic procedures are carried out within normal room lighting conditions, accordingly, a primary requirement for any computed radiography x-ray cassette is to shield the storage phosphor from exposure by ambient light.
In normal computer radiographic procedures, the portion of the patient being imaged is placed between an x-ray source, and an x-ray cassette containing the storage phosphor element used to record incident X-rays. When the patient is irradiated, the x-ray beam is selectively attenuated by differences in the density of the patient tissue and bone structure. When the imaging beam strikes the x-ray cassette, a portion of the beam is further attenuated by the front panel of the cassette, the remaining x-ray beam then passes through the storage phosphor, causing a direct exposure to the surface of the storage phosphor. The storage phosphor can then be scanned by a laser to yield the resulting image. The residual small portion of the imaging beam is further attenuated by a thin sheet of lead foil applied to the interior face of the back panel, or the back side of the storage phosphor screen, leaving only a very small fraction of the original beam to escape through the back surface of the cassette.
To minimize irradiation of the patient, it is desirable to minimize the attenuation of the imaging beam, thus, a third requirement for x-ray cassette design is the utilization, for the front panels of such cassettes, of materials which do not substantially attenuate an x-ray beam. Additionally, such front panels must provide for uniform attenuation, so as to not alter the pattern of attenuation created by the patient.
Materials which are lower in atomic weight are more transparent to x-rays, thus beryllium would be an excellent material for x-ray cassettes, were it not for its great cost, and toxicity. Thermoplastic materials which are primarily compounds of carbon, hydrogen, and oxygen are also suitable, although they generally lack structural strength and stiffness required. Carbon fiber reinforced thermoset resins are commonly used, in spite of their great cost. Aluminum is generally suitable, in the wavelengths used for general radiography, however it is heavy as well as having undesirable characteristics at longer wavelengths.
From the foregoing description, it should be apparent that it would be desirable to have an x-ray cassette that can be widely utilized to provide lower cost materials, and having equally high image quality, while minimizing the imaging beam attenuation of the front panel of the cassette, to reduce patient irradiation levels. It would be further desirable to provide a cassette which is structurally equivalent to the current cassette; and which is lighter in weight, for portability.